To make these grammatical elements parallel, change the part of speech in the third item to match the others:

Correct

The article is long, confusing, and insulting to my intelligence.

Insulting, like the other items in the series, is an adjective.

A common source of parallelism trouble is the expression as…as. For example:

Incorrect

It is as tall or taller than the Empire State Building.

This is an error because the elements on either side of the coordinating conjunction or are not parallel. You can say, It is…taller than the Empire State Building, but you cannot say, It is as tall…than the Empire State Building. To correct the error, add another as to the sentence:

Correct

It is as tall as or taller than the Empire State Building.

The revised version is the correct shorthand equivalent of writing, It is as tall as the Empire State Building, or it is taller than the Empire State Building.

A final example of a parallelism problem involves the frequently misused correlative conjunction not only…but also:

Incorrect

The book is not only boring but also costs a lot.

Whenever you use not only…but also, the grammatical elements linked by the expression must be the same — for instance, two nouns, two predicates, and so on. In the example above, boring is an adjective. Therefore, what follows but also should also be an adjective. In the sentence’s current form, however, what follows is instead a verb: costs.